Tube amp troubles

Friends,

I am using an Ayon integrated tube amp. During a recent move, I had to take out all the tubes and when I put them back, I mixed up the low-power pre-amp tubes. I’m only mentioning this is as a context - I don’t think it has much to do with the actual problem I have. 

When I switch on the amp and start using it after some warm-up (the amp indicates when it’s ready for a load), everything works fine for about an hour. Then, the left channel goes totally mute. If I switch off the amp and let it rest for a few minutes (but not long enough to cool down entirely), it works again (after powering it back on) for a while - and then the same thing happens. The problem occurs regardless of the input, volume, etc. 

If you have any idea what this could be, that would be very helpful - I would like to avoid shipping this 48 kg beast to the only authorized dealer.

Also, if you know of anyone in the greater DC area who repairs tube amps, that may come handy, too, in case there is no quick remedy.

Thank you all,

Csaba